This is all theoretical, so going by what other octo-keepers have observed may be more useful, but I think this info has helped a few folks get some more viewing of their nocturnal octos:
For the most part, octos don't like bright light at all... Although blue light is closest to what they'd see in their natural environments, even the species that aren't nocturnal are usually more active at dawn and dusk than the brightest part of the day. Also, an octopuses eyes are sensitive to different color ranges than ours are, so they see blue light as brighter than we do, and red light as much dimmer, and maybe not at all. If you're interested in watching while your octopus is most comfortable, increasing the red light and decreasing the blue will be the best tradeoff for making the light seem dim to your octo while still allowing you to see it. Some people have had some luck with using red LED flashlights in an otherwise dark room... LEDs in particular are probably good for this, since red light bulbs are white or yellow with a red filter, so they sometimes seem to appear bright to an octo. There are apparently also "Red UV lights," which is actually impossible (UV is bluer-than-blue, so it can't be red) which probably are UV lights tuned to make some corals and things glow red... I'd expect those to also look bright to an octo.